A magnesia carbon brick (hereunder, this is called as “MgO—C brick”) is a brick comprising magnesia and graphite as main aggregates and having excellent corrosion resistance and spall resistance, whereby it is widely used as a lining material of the whole of furnaces including a converter.
As the operation condition of a refining vessel is becoming increasingly severer in recent years, an MgO—C brick having more enhanced durability has been required. As a barometer of the durability of the Mg—O brick, the oxidation resistance and the corrosion resistance may be mentioned. In order to enhance these characteristics, making the MgO—C brick denser whereby lowering the air permeability with the outside air and suppressing penetration of slag and molten iron is effective. In the past, in order to make the MgO—C brick structure denser, attempts including improvement of the blending recipe thereof and introduction of a high power vacuum pressing machine have been made whereby achieving significantly low porosity thereof; and at the same time, it was confirmed that the durability thereof could be improved thereby significantly contributing to reduction of the unit consumptions of the refractories.
On the other hand, the evaluation technology of the MgO—C brick has been improved. In the past, mainly the characteristics have been evaluated after the MgO—C brick is dried, whereas recently the characteristics are evaluated after it is reductively fired. According to this, even if the apparent porosity is 3% or less after drying, it can occasionally reach 10%, or even higher after the reductive firing at 1400° C. for 3 hours, whereby the approximating value can be obtained to that of the brick after used. That is, the state approximating more actual use state of the sample can be obtained if the sample is reductively fired in advance as compared with otherwise, so that this can be judged effective as the improvement index of the material.
It has been known that the denseness of the MgO—C brick changes with the difference in the grain size distribution of the magnesia raw material; and for example, Patent Document 1 proposes the dense MgO—C brick whose oxidation resistance, corrosion resistance, and thermal strength can be improved by containing therein 30 to 45% by weight of the medium-sized particles having the size of 1 to 0.2 mm and 15 to 25% by weight of the fine particles having the size of 0.2 mm or less.
In addition, Patent Document 2 proposes the highly durable MgO—C brick whose structure deterioration can be suppressed so that the corrosion resistance thereof at the early stage in operation can be kept. This Patent Document 2 points out, as the factor of the structure deterioration of the MgO—C brick, the oxidation and reduction reactions between the magnesia and the carbon caused by the heat which is received in operation; and as the measure for improvement thereof, reduction of the amount of fine powders of the magnesia in the raw material mixture is proposed.